The midfielder made his debut in defeat to Napoli.
He said: "I know Muntari and De Jong very well, and I'm happy to be in the same team with team. I got to know De Jong when we were both playing in England.
"I also know Kakà well because we played together at Real Madrid. I'm very happy to see them all again. It's nice to have them here, and I'm happy to have joined the Milan family. It's great to be in a squad with these kind of players."